As a process for production of colored fibers, there has hitherto been known a spinning technique in which a raw material polymer is mixed with various dyes, pigments or inorganic compounds such as carbon black and titanium oxide. This technique is widely employed as the so-called "mass coloration". Alternatively, colored fibers can also be obtained by various conventional staining techniques after the formation of fibers.
In cases where mass coloration is employed for production of ultra-high molecular weight polyethylene fibers, various additives such as inorganic compounds are added to a solution of the raw material polymer, and these additives act as defects in the crystallization of ultra-high molecular weight polyethylene for development of high tenacity, thereby causing adverse effects such as tenacity lowering. To avoid such a problem, fine particles of inorganic compounds or organic pigments may be used for mass coloration. In general, however, fine particles have a particle size distribution which may usually include the range of particular particle sizes becoming defects, and the use of fine particles having a uniform particle size is disadvantageous in most cases from an economical point of view. Moreover, when fine particles of pigments are used, filter clogging and fiber breaking will occur in the spinning step, which is not fit for continuous operation.
In cases where a staining technique after the fiber formation is employed for production of colored high-tenacity fibers of ultra-high molecular weight polyethylene, it is difficult to achieve staining with sufficient fastness because polyethylene has a simple chemical structure and extremely high crystallinity.